Such a similar method is known from the American patent specification U.S. Pat. No. 5,633,212 and is used to apply oxide layers to semiconductor substrates such as silicon wafers. One or more wafers are placed in a wafer rack and are then transferred to an oven. During the insertion, the atmosphere is inert and will consist mainly of nitrogen.
In this American patent specification U.S. Pat. No. 5,633,212, the so called `wet oxidation` is described. Water vapor must be made and this is done by fitting a combustion chamber upstream of the reactor. There, oxygen and hydrogen are mixed with each other and combusted at a raised temperature into water vapor.
To control this combustion, the American patent specification in question suggests that after inserting the wafers into the oven, an oxygen atmosphere should first be introduced into the combustion chamber. To that end, the nitrogen supply is completely cut off, from one moment to the next, and the supply of oxygen is gradually started until the intended working maximum is reached. The supply of hydrogen is then gradually started up and, after a certain hydrogen flow is reached, the combustion takes place. During the increase of the oxygen concentration before the introduction of hydrogen, no oxidizing occurs due to the comparatively high concentration of nitrogen in the oven. Only after the abrupt cutting off of the nitrogen supply does the oxidation reaction actually start in the oven. The combustion is then already taking place in the desired manner in the pre-combustion chamber.
In the state of the art there is on the one hand the tendency to decrease the thickness of the oxide layer and on the other hand there is the tendency to increase the dimensions of every wafer. It has been shown that, with these two steps, the application of oil layers becomes critical. By increasing the dimensions of the wafers, increased non-uniform oxidation will occur in the radial direction. In general this is not critical except when a thin layer of oxide has to be applied. In such a case, control of the process and of the oxidized thickness is of great importance. It has also been observed that with a vertical oven where several wafers are inserted that lie vertically behind each other, differences occur in the oxide growth of the uppermost and lowermost wafers. Such differences are also undesirable.
This problem does not only hold for oxidation but for many other processes wherein changes in the process conditions take place, that is, when one process gas is switched over to another process gas.